1. Field of the Invention
This invention relates to a filtration roving with selected portions twisted more or less than other portions, to a new device for making such roving, to a new method for making such roving, and to wound filters employing such roving.
2. Description of Prior Art
Rovings are produced by spinning staple, textile fibers. In a particular type of unique spinning machine, fibers are fed to the area between two spinning drums. The drums rotate in the same direction at very high speed. The fibers cannot keep up with the drums and a twist is imparted to it creating a roving. Suction inserts in the drums themselves are utilized to pull the fibers down into the space between the drums, increasing the friction on the fibers thereby increasing the amount of twist.
To change the amount of twist various efforts have been made such as changing the amount of suction pressure constantly throughout the drums or by altering the drum speed. Although these methods do provide some adjustment in overall roving twist, they do not permit the selective variation of the twist of isolated sections of a roving.
For use in filtration, such as shown in U.S. Pats. Nos. 4,225,442; 3,624,779; 3,356,226; 3,319,793; and 2,368,216 an ideal roving would have a high enough amount of twist to impart sufficient strength to permit processing in the various winding mechanisms, but it would also have a low enough amount of twist to give sufficient bulk to the roving so that the desired filtration can be accomplished. It has generally been believed that there is a one-to-one trade off between bulk (low twist) and strength (high twist) and that some advantages of either must be compromised in any particular roving.
Bulk is important because increased roving bulk increases the amount of void space within a filter made with the roving in which particles can be trapped. Increased bulk reduces the amount of roving necessary to make a particular filter. Strength is important because a weak roving results in filter winding machine downtime when a roving breaks.